The present invention relates generally to digital audio broadcasting (DAB) and other techniques for transmitting information, and more particularly to techniques for implementing hybrid in-band on-channel (IBOC) systems for DAB and other applications.
The explosive growth of digital communications technology has resulted in an ever-increasing demand for bandwidth for communicating digital audio information, video information and/or data. For example, to efficiently utilize bandwidth to communicate digital audio information, a perceptual audio coding (PAC) technique has been developed. For details on the PAC technique, one may refer to U.S. Pat. No. 5,285,498 issued Feb. 8, 1994 to Johnston; and U.S. Pat. No. 5,040,217 issued Aug. 13, 1991 to Brandenburg et al., both of which are incorporated by reference herein. In accordance with such a PAC technique, each of a succession of time domain blocks of an audio signal representing audio information is coded in the frequency domain. Specifically, the frequency domain representation of each block is divided into coder bands, each of which is individually coded, based on psycho-acoustic criteria, in such a way that the audio information is significantly compressed, thereby requiring a smaller number of bits to represent the audio information than would be the case if the audio information were represented in a more simplistic digital format, such as the PCM format.
Recently, the industry turned its focus to the idea of utilizing preexisting analog amplitude-modulation (AM) frequency band more efficiently to accommodate digital communications as well. However, it is required that any adjustment to the AM band to provide the additional capacity for digital communications does not significantly affect the analog AM signals currently generated by radio stations on the same band for AM radio broadcast. In the United States, adjacent geographic areas covered by AM radio broadcast are assigned different AM carrier frequencies, which are at least 20 kHz apart. Specifically, when they are exactly 20 kHz apart, the AM carrier assigned to the adjacent area is referred to as a xe2x80x9csecond adjacent carrier.xe2x80x9d Similarly, when they are 10 kHz apart, the AM carrier assigned to the adjacent area is referred to as a xe2x80x9cfirst adjacent carrier.xe2x80x9d
An in-band on channel AM (IBOC-AM) (also known as xe2x80x9chybrid IBOC-AMxe2x80x9d) scheme utilizing bandwidth of the AM band to communicate digital audio information has been proposed. In accordance with the proposed scheme, digitally modulated signals representing the audio information populate, e.g., a 30 kHz digital band centered at an analog host AM carrier. The power levels of the spectrums of the digitally modulated signals are allowed to be equally high across a 10 kHz subband in the digital band on each end thereof.
However, in implementation, it is likely that two such IBOC-AM schemes would be respectively employed in two adjacent areas, to which the host AM carriers assigned are 20 kHz apart. In that case, the 30 kHz digital bands for digital communications centered at the respective host AM carriers overlap each other by 10 kHz, thereby causing undesirable xe2x80x9cadjacent channel interferencexe2x80x9d to each area. In particular, such interference is referred to as xe2x80x9csecond adjacent channel interference,xe2x80x9d as the dominant interfering carrier in this instance consists of a second adjacent carrier. For example, the second adjacent channel interference degrades the digital communications in each of the adjacent areas, especially in the parts of the areas which are close to their common border. Similar concerns arise in other types of IBOC systems, e.g., frequency-modulation (FM) IBOC systems, also known as IBOC-FM systems or hybrid IBOC-FM systems, satellite broadcasting systems, Internet radio systems, TV broadcasting systems, etc.
Accordingly, there exists a need for a technique, e.g., based on the PAC technique, for effectively utilizing an existing transmission band, e.g., an AM, FM or other band, for digital communications and treating adjacent channel interference in adjacent areas where IBOC schemes are employed.
The present invention provides methods and apparatus for multistream transmission and/or reception of information in IBOC digital audio broadcasting and other applications. In accordance with the invention, multiple bit streams are generated from an information signal, and the bit streams are transmitted using frequency bands associated with a host carrier signal, e.g., an AM or FM host carrier signal. The manner in which the multiple bit streams are generated and transmitted may be based on factors such as, e.g., multidescniptive coding, a core/enhancement type of embedded coding, a lower basic coding rate in one frequency band relative to another frequency band, bit error sensitivity classification for unequal error protection (UEP), a non-uniform power profile on the bands, an increased total frequency band power, and an increase in frequency band and bit stream time diversity by introducing delay between bit streams in different bands and/or within the same band. The individual bit streams may be encoded using an outer code, e.g., a CRC code, RS code, BCH code, or other linear block code, and an inner code, e.g., a convolutional code, turbo code, or trellis coded modulation.
In an illustrative embodiment, a set of bit streams are generated from an audio information signal. The set of bit streams may be, e.g., a total of four bit streams generated by separating each of two multiple description bit streams, corresponding to separate representations of the audio information signal, into first and second class bit streams. The first and second class bit streams associated with the first multiple description bit stream may then be transmitted in respective first and second subbands of a first sideband of an FM host carrier, while the first and second class bit streams associated with the second multiple description bit stream are transmitted in respective first and second subbands of a second sideband of the FM host carrier. The first class bit streams may be provided with a different level of error protection than the second class bit streams, e.g., by utilizing different portions of a non-uniform power profile for the corresponding subbands, or by placement of the bit streams in subbands having different susceptibility to interference. Delay may be introduced between at least a subset of the four bit streams in order to provide improved performance, e.g., in the presence of fading.
The invention provides a number of other significant advantages over conventional systems, including, for example, improved coverage area and reduced memory requirements. The invention may be implemented in numerous applications, such as simultaneous multiple program listening and/or recording, simultaneous delivery of audio and data, etc. In addition, one or more of the techniques of the invention can be applied to other types of digital information, including, for example, speech, data, video and image information. Moreover, the invention is applicable not only to perceptual coders but also to other types of source encoders using other compression techniques operating over a wide range of bit rates, and can be used with transmission channels other than radio broadcasting channels.